This invention relates generally to leak detection systems for testing containers, and more particularly to a system in which the integrity of a container is dry-tested in a closed chamber.
In testing steel drums, large plastic or metal receptacles or other large containers, existing techniques generally involve a wet test procedure. Thus the common practice is to first close the container and then submerge the closed container in a water tank to see if bubbles emerge therefrom, thereby indicating the presence of a leak. Another practical approach is to close and pressurize the container and to thereafter spray a soap solution thereover, soap bubbles being generated on the surface of the container should there be a leak therein.
Whether the wet test is conducted under water or by means of soap bubbles, in either case careful visual observation is required to find leaks. A gross leak will be immediately apparent, but a minute fissure, which nevertheless renders the container unacceptable, calls for more careful scrutiny. The practical drawback in wet testing containers is that the human observer, after an hour or more of tedious testing on a production line basis, may become fatigued, bored or inattentive, and may fail in some instances to note the presence of small bubbles indicative of a leak.
Moreover, because present production rates for large (55-gallon) steel containers are one drum per six seconds, the human operator is unable to observe or test the ends of the drum which are clamped and obscured, or to see the chime seams around the drum, for the drums are held in only one position. Accordingly, many leaks escape the view of even the diligent human observer when wet testing on a production line basis.
Also known are so-called dry testing techniques wherein the drum or container is placed within a test chamber which is hermetically sealed, after which the drum is pressurized with air through its bung hole. No air from the interior of the pressurized drum can flow into the test chamber unless there is a leak in the drum, the existence of this leak being determined by sensing the pressure within the test chamber. Should no leak exist, the test chamber will remain at its normal pressure level, but if there is a leak, then the chamber pressure will rise above the normal level because of the transfer of pressurized air from the interior of the container to the chamber interior.
The difficulty with this known dry testing technique and the reason why it is especially inadequate in checking large containers is that leakage from the pressurized container into the test chamber is by no means the only factor giving rise to an increase in chamber pressure, for several other factors come into play.
Thus should the dry tester be zero set at a given chamber atmospheric temperature, a fall in temperature within the chamber may cause a leaking container to improperly pass the leak test. Though the leak produces a rise in test chamber pressure, a temperature drop in the atmosphere within the chamber reduces this pressure and the resultant pressure reading will therefore not reflect the leak. On the other hand, a rise in the temperature of the chamber atmosphere may cause good containers to appear leaky, particularly since the influence of a small change in temperature on the test chamber pressure is much larger than the effect of a small leak. Hence with existing dry testing systems, thermal effects act to totally mask small leaks.
Furthermore, a change in pressure within the test chamber will also result from a change in its operating volume. The operating volume of the test chamber is determined by the overall volume of the chamber minus the volume of the container therein under test. The test chamber itself is designed to be extremely rigid, hence its overall volume is constant, but the containers being tested are not rigid and, when pressurized, these containers "breathe" or change in volume with even the minutest change in internal pressure. Thus the pressure in the test chamber may reflect a change in the operating volume of the chamber rather than a leak in the container under test.
An additional problem is the influence of the air supply from the compressor feeding air into the drum being tested. This air varies in temperature which, in turn, changes the size of the drum as it is heated during a compression cycle or cooled by the varying temperature of the compressed air. One must also take into account the varying temperature in any typical production plant such as cold mornings, and radiation during warm afternoons which tend to heat up the test chamber and cause thermal effects that interfere with the detection of small leaks.
Moreover, the steel drums to be tested can vary in temperature from cold to very hot; these varying temperature conditions resulting from the design of the production line, by the seaming operations, by baking ovens, and by stoppages in the production line which tend to cool otherwise heated drums. As a consequence, the drums arrive at the test chamber at temperatures that vary tremendously.
Still another complication that has militated against the success of dry-testing systems of the type heretofore known is that large steel containers are not uniformly fabricated of the same gauge steel and, depending on the changing gauge, successive containers will vary in their physical growth and distortion rates under pressure during test.
The problems outlined above which are involved in testing containers become aggravated with an increase in container size. Thus for steel drums which are usually manufactured in capacities ranging from 1 to 55 gallons, the greatest difficulties are encountered in testing 55-gallon containers.
Or to express this relationship in positive terms, any dry-testing system capable of adequately testing 55-gallon containers will also be capable of testing smaller containers more efficiently and more accurately. But the fact remains that no dry-testing system of the type heretofore known has been capable of satisfactorily testing 55-gallon containers for leaks.